1. Field of the Invention
The present invention relates to a method of controlling thickness of the coated film on the web-like member by the roll coater, and more particularly to a method of controlling thickness of the coated film on the web-like member by the roll coater, wherein, when coating is performed continuously on the web-like member such as cold-rolled steel plate by the roll coater, the film thickness can be controlled at high accuracy.
2. Prior Art
With the steel sheets, in order to improve the performance such as corrosion resistance, for example, there has been commonly practiced coating of chrome, resin and the like is made on a galvanized steel sheet.
The above-described coating onto the steel sheet is performed such that the steel sheet paid off from a payoff reel in an inlet facility, while being continuously conveyed, passes through processes including a degreasing process, a coating process by use of a roll coater and a drying process by use of an oven (same process is repeated as necessary). Then, the steel sheet after the coating is adapted to be wound up by a wound-up device in an outlet facility.
In general, the roll coater used for continuous coating of the steel sheet includes a steel pickup roll for picking up a paint in a paint pan and a rubber-lined applicator roll for receiving the paint from the pickup roll and for transferring and coating the paint onto the surface of the steel sheet. When the coating is performed by use of this roll coater, the control of the thickness of the coated film is performed by suitably controlling the circumferential speeds of rolls, an urging force between the pickup roll and the applicator roll and an urging force between the steel sheet and the applicator roll with respect to a conveying speed of the steel sheet.
Now, in the recent years, the coated steel sheets have been used for the wider application in the domestic electrical equipment, motor vehicles, building materials and the like, whereby the material quality required by the demands such as the improved anticorrosion performance is raised and the accuracy of the thickness of the coated film comes to be very strict.
Heretofore, as the method of controlling the thickness of the coated film in the coating by use of the roll coater, there has been known a method of controlling the urging force between the pickup roll and the applicator roll and the urging force between the steel sheet and the applicator roll at predetermined values constantly as disclosed in Japanese Patent Laid-Open No. 6268/1983, and Patent Application Publications No. 56553/1985 and No. 41077/1987, for example, and a method of controlling the urging force between the pickup roll and the applicator roll in accordance with data concerning the relationship between the urging force and the thickness of coated film in the coating in the past as disclosed in Japanese Patent Laid Open No. 166959/1983 and Patent Application Publication No. 23225/1991, for example. However, as for the specific details of the methods of controlling and model equations, there is none described at all.
Furthermore, as another method of controlling the thickness of the coated film, such a method has been adopted that there is used a control equation based on only the circumferential speeds of the rolls and the moving speed of the steel sheet, and determined by the experimental regression.
Subsequently, explanation will be given of the case of continuously coating the rear surface of the web-like member, as in the case of both surfaces coating.
In general, in the process of continuously coating the both surfaces of the web-like member such as the steel sheet, as shown in FIG. 43, first, the front surface of a steel sheet S is coated by a first roll coater 10 at the first stage, subsequently, the rear surface is coated by a second roll coater 20, thereafter, the steel sheet is passed through a heating furnace 22 for drying and passed through a cooling furnace 24 for cooling, and delivered to the succeeding process. Incidentally, in the drawing, reference numeral 26 is a lift roll and 28 an outlet side fulcrum roll.
The above-described first roll coater 10 is constituted by a pickup roll 14 for picking up a paint P in a paint pan (paint pool), an applicator roll 16 for delivering part of the paint P picked up by the pickup roll 14 and transferring the paint onto the steel sheet S, and a backup roll 18 for urging the steel sheet S against the applicator roll 16 when the paint is transferred by the applicator roll 16. The above-described second roll coater 20 has the substantially same construction as the first roll coater 10 except for it has no backup roll.
When the both surfaces of the steel sheet S are coated, the steel sheet S is passed between the applicator roll 16 and the backup roll 18 in a state where the steel sheet S is wound around the backup roll 18 of the roll coater 10 to thereby coat the front surface, and subsequently, the rear surface is coated by passing the steel sheet S over the second roll coater 20, with the steel sheet S continuously conveyed in a catenary shape (in a suspended state) being pushed up by the applicator roll 16 of the second roll coater 20 from below.
At this time, the thickness of the film coated on the steel sheet S is greatly influenced by the urging force between the steel sheet S and the applicator roll 16, so that it becomes important to control the urging force to a target value. However, when the front surface is coated by the above-described first roll coater 10, the urging force between the steel sheet S and the applicator roll 16 can be positively controlled by the backup roll 18, whereas, when the rear surface is coated by the second roll coater 20, the urging force between the steel sheet S and the applicator roll 16 is determined by a tension acting on the steel sheet S, so that the urging force cannot be positively controlled.
Therefore, when the rear surface of the steel sheet S is coated by the above-described second roll coater 20, it is conceived that coating is made with the catenary shape being held constant. In order to hold the catenary shape constant as described above, in the steady state where the steel sheets S which are identical with one another are continuously coated, a unit tension (tension / sectional area of the steel sheet) should be controlled to a constant value. However, for example, when a preceding steel sheet and a succeeding steel sheet, which are different in size from each other, jointed together and have a sheet joint point (connecting portion) where the sectional areas of the both steel sheets differ from each other, are coated, at the time of the unsteady state where the sheet joint point passes through the catenary, the tension should be changed every moment to limit the fluctuations in the catenary shape to the minimum.
As a method of changing the tension with time when the sheet joint point passes through the catenary, there is a method disclosed in Japanese Patent Laid-Open No. 305750/1990, for example. This is a method wherein the tension in the catenary is calculated from tracking information of the joint position between the long materials being present in the catenary and being different in size or material quality, and information of the respective sizes and material qualities of the preceding material and the succeeding material which are present in front and back of the joint position, the height of the catenary is successively calculated in accordance with the joint position from the above-described tracking information, information of the sizes and material qualities and the calculated catenary tension, the catenary tension is monitored, and an excessive catenary tension or the fluctuations of the height of the lowest point of the catenary are suppressed in association with a deviation between the catenary height and the height of the catenary before the joint position enters the catenary, or by increasing or decreasing the speed of delivering the long materials when the catenary tension exceeds a predetermined value.
As described above, to hold constant the catenary shape, it is necessary to change the tension in accordance with the passing position of the sheet joint point when the sheet joint point of the steel sheets different in sectional area from each other passes through the catenary section, whereby the urging force between the steel sheet S and the applicator roll 16 is adapted to be changed every moment.
When the above-described urging force to the applicator roll 16 is changed as described above, as the coating conditions are typically shown in FIG. 44, when the urging force N.sub.A comes to be lower than the target value, the leak flow rate q.sub.L of the paint P escaping through without being transferred to the steel sheet S becomes higher, whereby the coating build-up onto the steel sheet S becomes lower than that at the time of the steady state. On the contrary, when the urging force N.sub.A comes to be higher than the target value, the leak flow rate q.sub.L is decreased, whereby the coating build-up onto the steel sheet S becomes higher.
When the coating build-up onto the steel sheet S changes as the urging force N.sub.A changes every moment, the thickness of the coated film should necessarily change; increasing or decreasing, thus resulting in defects of quality.
To explain this specifically, when the sheet joint point is passing through the catenary section, in spite of that the tension acting on the catenary section changes every moment so as to change the tension of the preceding steel sheet (the tension of the preceding steel sheet for holding the catenary shape) into the tension of the succeeding steel sheet (the tension of the succeeding steel sheet for holding the catenary shape), heretofore, upon passing of the sheet joint point over the applicator roll 16, a nip pressure (urging force) Np has been immediately set because the tension of the succeeding steel sheet is regarded as being realized, whereby, there has be such a problem that, when the preceding steel sheet (designated as the preceding material in the drawing) is larger in sectional area than the succeeding steel sheet (designated as the succeeding material in the drawing) as shown in FIG. 45, in spite of that chromate coating having a target film thickness of 50 mg/m.sup.2 is performed for example, the coating weight is changed greatly.
Then, heretofore, to prevent the change in the coating weight accompanied by the passing of the sheet joint point over the applicator roll 16, when the steel sheets greatly different in sectional area from each other are continuously coated, to compensate the difference in the ratio of sectional area therebetween, connecting steel sheet having values of sectional areas between the above-described steel sheets have been successively connected so as to be included within predetermined ratio of the sectional areas, so that the ratio of the sectional areas at the sheet joint point between the preceding steel sheet and the succeeding steel sheet can be limited to be low, thus avoiding the great change in coating weight.
However, with the method of controlling the urging forces between the rolls to the constant values as disclosed in the above-described Japanese Patent Laid-Open No. 6268/1983 and the like, the thickness of the coated film greatly changes according to the coating conditions such as the types of paints, the circumferential speed of the roll such as the applicator roll and the moving speeds of the steel sheets, so that it is difficult to control the thickness of the coated film to the constant value over the wide ranges of the coating conditions.
Further, rubber lined on the applicator roll is expanded and moistened by the thinner in the paint, whereby the elastic modulus (function of hardness) thereof is changed with time. Accordingly, when the urging force between the pickup roll and the applicator roll is set at a constant value, the expansion and moistening of the rubber progress, and the expansion and moistening are increased in degree, the surface pressure between the rolls is decreased, whereby the paint passing between the rolls is increased in quantities, thus increasing the thickness of the coated film.
Furthermore, in order to remove the influence due to the expansion and moistening of the rubber, it becomes necessary to perform the work for stabilizing the expansion and moistening (work for driving only the roll coater without performing the coating) for one or two hours until the expansion and moistening are stabilized, thus greatly deteriorating the production efficiency in this case.
Furthermore, with the method of controlling the urging force between the pickup roll and the applicator roll in accordance with the data in the past as disclosed in the above-described Japanese Patent Laid-Open No. 166959/1983 and the like, it is necessary to previously determine through experiments the conditions for setting at the predetermined values the types of paints, the degrees of dilution, the moving speeds of the steel sheets, the circumferential speeds of rolls, the target thickness of the coated film and the like, thus requiring much time and labor. Furthermore, in the case of this method, assurance is not obtained of that the change with time of the elastic modulus of the rubber of the applicator roll due to the expansion and moistening is always constant, so that no assurance can be obtained of the thickness of the coated film for the steel sheets used for motor vehicles, which require the strict accuracy in the thickness of the coated film.
Furthermore, with a method of controlling, wherein only the circumferential speed of the pickup roll, the circumferential speed of the applicator roll and the moving speeds of the steel sheets are evaluated and coefficients are determined experimentally through the regression, such problems are presented that a long period of time (one year, for example) is required before the stabilized control can be obtained, and moreover, the range of control to be applied is limited.
Further, for example, when the rear surface of the steel sheet is continuously coated by the second roll coater as shown in FIG. 43, if the method of reducing the ratio of sectional area between the preceding steel sheet and the succeeding steel sheet to a lower value is adopted, then, necessity for preparing a large quantity of connecting steel sheets occurs when the difference in the ratio of sectional area is large, and a period of time for threading the connecting steel sheets intervening becomes large, thus forming bottlenecks in improving the productivity.